Can I reprogram my 2015 Prius Two key to use as a 2nd key for my 2017 Prius Prime?

I don't think so. My 2015 Gen3 Two did not have 3 doors SKS like my (and your) 2017 PP Premium. It also had no remote A/C functionality. I don't think Primes ECU will recognize the Gen3 fob. You can find cheaper virginized fob for PP online for about ~$100. If you have TechStream, then you should be able to program it DIY. If you have to have someone program it, that alone will cost $100-200, but some dealers and/or locksmiths will not work with used virginized key fob, so you have to check before you buy used fob.

 

TechStream is diagnostic software used by Toyota. It is needed to access the ECU information and program it. The registration of key codes for a new fob is done by using TechStream. The official version of the software can be subscribed for fee and installed on your computer and used with a compatible OBDII adaptor, but it may not allow you to program a key fob IIRC. However, there are various versions of pirated software and knock-off mini-VCI OBDii adaptors out there. Try searching. I have read threads describing successful programming of a key fob using pirated software and knock-off mini-VCI OBDii adaptors on both Gen4 and PP. There are many Prius owners who use either pirated (illegal) TechStream or the official TechStream to do DIY diagnosis and customization.

That being said, If you have never heard of "TechStream" before, then you may be better off not trying programming the key DIY. In the worst-case scenario, you could brick the car completely. It can make the key now functioning not to start the car. Then you will have an additional cost of reprogramming the keys.